Filters are used in hot cells or areas in which work is performed on equipment or substances that produce significant levels of radiation that may be threatening to humans. For instance, in hot cells, the work that is done is performed by electromechanical manipulators, master-slave manipulators, and overhead cranes, operated from remote locations by humans who are shielded from the radiation. Air in hot cells is circulated and filtered to prevent the release of radioactive materials to the environment. It is also necessary to filter the gases emanating from process equipment in hot cells. Such air or other gas is filtered to remove particulate matter that may be radioactive or hazardous. The filters and their housings are thus located in the hot cell and are radioactive or contaminated with radioactive materials or other hazardous elements.
A method of recovering hazardous wastes collected on a phenolic resin filter used to filter fluid systems is disclosed in U.S. Pat. No. 4,995,916 issued Feb. 26, 1991. This method dissolves the phenolic fiber material and particulate contained in the filter cartridge by 8-12 molar nitric acid at temperatures of 160.degree.-190.degree. F. This patent is assigned to the U.S. Department of Energy.
HEPA filters are disclosed in U.S. Pat. No. 4,773,922 issued on Sept. 12, 1988, and also assigned to the U.S. Department of Energy.
A HEPA filter or complex of filters must be removed and replaced when particulate buildup creates an excessive pressure drop in the off-gas system. After replacement, the used filters require some form of regulated disposal method that prevents spread of the radioactive or hazardous particulates. Spent HEPA filters from nuclear facilities are typically classified as high-level, transuranic (TRU), and mixed waste, which is impossible to dispose of without processing.
Because of the difference of the filter media from the HEPA filter of the present process, the above phenolic filter process is not appropriate for the HEPA filter dissolution. HEPA filter materials are primarily silicon (SiO.sub.2), boron (B.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), and KEVLAR.TM..
As an alternative, a HEPA filter leach system may be used to "clean" the filter element; however, leaching tests of these HEPA filters in some cases has failed to remove enough mercury for the filters to meet EPA toxic concentration leach procedure (TCLP) test limits. Accordingly, it would be desirable to have an alternative to leaching. The present invention, filter media dissolution in an HF mixture, has been developed as a disposal method.